1. Field of the Invention
The present invention relates to a magnetic recording medium, and more specifically a magnetic recording medium having a metal thin film type magnetic layer.
2. Description of the Related Art
Conventionally, magnetic recording mediums of the type in which magnetic layers are formed by application are generally widely employed. These mediums are generally manufactured by applying and drying magnetic materials that include: a binder material such as vinyl chloride-vinyl acetate copolymer, polyester resins and polyurethane resins; and a magnetic material and other additives dispersed into the binder material.
On the other hand, so called metal thin film type magnetic recording mediums are proposed, that are manufactured by forming films of ferromagnetic metal materials such as Coxe2x80x94Ni alloys and the like onto a film shaped base material such as polyethylene terephthalate (PET) and the like via vacuum thin film forming technology (such as vacuum deposition, sputtering, ion-plating and the like). Such metal thin film type magnetic recording mediums have been practically utilized, because of the following advantages: the metal thin film type magnetic recording mediums can be designed to have very thin in thickness as compared with the conventional applying type magnetic recording mediums; the metal thin film type magnetic recording mediums has better electromagnetic conversion characteristics at short wave due to its better magnetic retention, better squareness ratio and the like; packing density of the magnetic materials is higher due to the absence of the nonmagnetic material such as organic binders and the like in the magnetic layer, thereby improving storage density.
However, since the metal thin film type magnetic recording medium is formed by directly depositing magnetic metals onto the base, the metal thin film type magnetic recording medium has very smooth magnetic layer surface and has substantially larger contact surface area and since the magnetic metal itself does not provide lubricity, the friction coefficient of the surface may increase to adversely affect the tracking stability and the friction stability. Accordingly, a lubricating layer is proposed for reducing the surface friction coefficient of the metal thin film type magnetic recording medium to improve the tracking ability. The lubricating layer may be formed by applying a lubricant on the protective layer of hard carbon film and the like that is formed onto the surface of the magnetic layer.
As such, it is critical to monitor the condition for forming the lubricating layer, i.e., the condition after applying the lubricant, for the purpose of ensuring the tracking stability, friction stability and the like of the metal thin film type magnetic recording medium. In such cases, ESCA (Electron Spectroscopy for Chemical Analysis) is often employed for monitoring quantity of the lubricant applied on the magnetic layer. For example, in the case of organic lubricants containing fluorine atoms, the quantity of the applied lubricant can be monitored by monitoring a number counted from the ESCA data that are obtainable by performing ESCA measurements with the fluorine chemical shift after forming the lubricating layer.
However, ESCA does not provide the Information on the distribution of the lubricant, though ESCA successfully provides the quantitative conditions of the formed lubricant since ESCA provides the quantitative measurements, and therefore a research focusing on the distribution of the lubricant across the surface of the magnetic recording mediums at the time of forming the lubricating layer has not been achieved.
Consequently, an object of the invention is to provide an improved metal thin film type magnetic recording medium having improved tracking stability and improved friction stability by providing an improvement in the distribution of the lubricant contained across the surface of the lubricating layer.
The present inventors have found, as a result of various investigations carried out for the purpose of solving the problems indicated above, methods for readily measuring and controlling the distribution of the lubricant across the lubricating layer by using low cost equipments, and further the present inventors have found that the metal thin film type magnetic recording medium having improved tracking stability and improved friction stability by controlling the distribution of the lubricant across the lubricating layer with the present method to stabilizing the lubricant covering ratio onto the surface of the lubricating layer to an appropriate level, and thus the present invention is completed.
That is, a magnetic recording medium according to the present invention is the magnetic recording medium at least having, in turn, a metal magnetic layer, a protective layer and a lubricating layer on one surface of a nonmagnetic base, wherein the covering ratio of a surface of the lubricating layer with the lubricant is 65-75%.
According to one aspect of the present invention, it is preferable that the lubricating layer is formed by applying a mixture of solvents containing at least three solvents selected from three groups, that is from at least one from group (1): ketone solvents having a boiling point of 40-130xc2x0 C., at least one from group (2): aliphatic hydrocarbon solvents having 4 carbons-10 carbons, and at least one from group (3) alcoholic solvents having equal or less than 6 carbons. More particularly, the mixture ratio of these solvents of respective groups is preferably selected to be: (1) ketone solvents 5-20% wt.; (2) aliphatic hydrocarbon solvents 10-40% wt.; and (3) alcoholic solvents 40-80% wt.
Further, the covering ratio can be preferably measured by using conductive atomic force microscope (AFM), and the protective layer preferably includes a compound having hard carbon film and the lubricating layer preferably includes a lubricant of a compound having fluorine atom.